Hello Robert,
Well, I think the feedback has been pretty clear, honestly. Here's
what I'm unhappy about: I can't understand what these options are
actually doing.
We can try to improve the documentation, once more!
However, ISTM that it is not the purpose of pgbench documentation to be a
primer about what is an exponential or gaussian distribution, so the idea
would yet be to have a relatively compact explanation, and that the
interested but clueless reader would document h..self from wikipedia or a
text book or a friend or a math teacher (who could be a friend as well:-).
[nttcom@localhost postgresql]$ contrib/pgbench/pgbench --exponential=10
starting vacuum...end.
transaction type: Exponential distribution TPC-B (sort of)
scaling factor: 1
exponential threshold: 10.00000
decile percents: 63.2% 23.3% 8.6% 3.1% 1.2% 0.4% 0.2% 0.1% 0.0% 0.0%
highest/lowest percent of the range: 9.5% 0.0%
I don't have a clue what that means. None.
Maybe we could add in front of the decile/percent
"distribution of increasing account key values selected by pgbench:"
Here is an example of an explanation that would make sense to me.
This is not the actual behavior of your patch, I'm quite sure, so this
is just an example of the *kind* of explanation that I think is
needed:
This is more or less the approximate behavior of the patch, but for 1% of
the range, not 50%. However I'm not sure that the current documentation is
so bad.
The --exponential option causes pgbench to select lower-numbered
account IDs exponentially more frequently than higher-numbered account
IDs. The argument to --exponential controls the strength of the
preference for lower-numbered account IDs, with a smaller value
indicating a stronger preference. Specifically, it is the percentage
of the total number of account IDs which will receive half the total
accesses. For example, with --exponential=10, half the accesses will
be to the smallest 10 percent of the account IDs; half the remaining
accesses will be to the next-smallest 10 percent of account IDs, and
so on. --exponential=50 therefore represents a completely flat
distribution; larger values are not allowed.
--
Fabien.
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